1. Field of the Invention
The present invention relates to a piezoelectric ceramic composition suitable for resonators and the like, a manufacturing method for the piezoelectric ceramic composition, and a piezoelectric element.
2. Description of the Related Art
Piezoelectric ceramic compositions are widely used as the materials for the piezoelectric elements for use in resonators, filters, actuators, ignition elements, ultrasonic motors and the like. Most of the piezoelectric ceramic compositions now being put in practical use are constituted with ferroelectrics having the perovskite structure such as PZT (the PbZrO3—PbTiO3 solid solution) based or PT (PbTiO3) based ferroelectrics having the tetragonal system or the rhombohedral system at around room temperature.
In these years, concurrently with the miniaturization of electronic devices including communication apparatuses, surface mounting of parts has been rapidly progressed. In surface mounting of parts, a piezoelectric element preliminarily mounted on a substrate is soldered. It is unpreferable that the properties (for example, the resonant frequency, oscillation frequency and the like) of the piezoelectric element are largely deviated from the initial properties after the soldering treatment involving heating. Accordingly, various investigations have been made for the purpose of improving the heat resisting properties of the piezoelectric ceramic compositions (for example, see: Japanese Patent Laid-Open No. 8-333158(hereinafter referred to as Patent Document 1), Japanese Patent Laid-Open No. 8-333159 (hereafter referred to as Patent Document 2), Japanese Patent Laid-Open No.8-333160 (hereafter referred to as Patent Document 3), Japanese Patent Laid-Open No. 11-209176 (hereafter referred to as Patent Document 4), Japanese Patent Laid-Open No. 11-322419 (hereafter referred to as Patent Document 5), and Japanese Patent Laid-Open No. 11-322420 (hereafter referred to as Patent Document 6)).
For example, Patent Documents 1 to 3 disclose that the sintered bodies of PZT based piezoelectric ceramic compositions are subjected to polarization, annealed at a temperature of 0.4 times or more and 0.8 times or less the Curie temperature for 1 hour or more, thereafter subjected to aging treatment for 48 hours or more at a normal temperature, and then polished to be formed into piezoelectric resonators.
Patent Document 4 discloses that a sintered body is poled by applying a direct current electric field of 3.5 kV/mm or more at temperatures within the range from 130 to 180° C., and is subjected to heat treatment at 220 to 280° C. while the polarizing electrodes of the poled sintered body are short circuited; thus, there can be obtained a piezoelectric ceramic composition which is small in the variations of the electric properties even at a temperature around 280° C. encountered in the mounting by soldering and also small in the resonant frequency variation caused by a temperature cycle.
Patent Documents 5 and 6 disclose that a heat treatment is conducted under the conditions such that the product between the heat treatment temperature and the heat treatment time is 1800 (° C.·hour) or more at a temperature of 150 to 250° C. while the polarizing electrodes are being short circuited.
According to the methods described in Patent Documents 1 to 3, the resonant frequency variation rate of a resonator can be made smaller. However, the methods described in Patent Documents 1 to 3 require at least 49 hours in total including the annealing time and the time for the aging treatment subsequent to annealing, and accordingly there is a problem involving the productivity. Similarly, in the methods described in Patent Documents 5 and 6, the time required for heat treatment is long (in the examples involved, a heat treatment was carried out at 200° C. for 15 hours), and moreover, the resonant frequency change rate observed in a test for heat resisting properties is still at a high level.
Additionally, in Patent Document 4, the heat resisting properties of a resonator is judged to be satisfactory when the rate of change in electromechanical coupling factor k15 of the resonator, that is, Δk15 is 5% or less in absolute value and the rate of change in resonant frequency Fr of the resonator, that is ΔFr is 0.3% or less in absolute value. In Patent Document 4, these values of the resonator have been measured at an elapsed time of 30 minutes after the resonator was kept on a hot plate at 280° C. for 1 minute. However, a piezoelectric ceramic composition more excellent in heat resisting properties is demanded.